How can I get rid of the feeling of guilt for having a bad opinion of my sister, despite my repentance and seeking forgiveness?

The questioner erred by having ill thoughts about her sister without proof. She must sincerely repent, and frequently seek forgiveness for her sister, supplicate for her, and respect her rights; because Allah Almighty says: {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} And the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the most untruthful of speech." And whenever a Muslim has an evil thought about another Muslim, he should increase his consideration for him and pray for his well-being, for that enrages Satan.